The preparation of water-absorbent polymers is, for example, summarized in “Modern Superabsorbent Polymer Technology” F. L. Buchholz and A. T. Graham, Wiley-VCH, 1998, or in Ullmann's Encyclopedia Industrial Chemistry, 6th ed. vol. 35 pp 73-103. The preferred preparation process is the solution or gel polymerization. When using this technology usually a monomer mixture is prepared which is discontinuously neutralized and then transferred into the polymerization reactor and is then discontinuously or continuously polymerized to form a polymer gel which is in case of a stirred polymerization comminuted. The polymer gel is subsequently dried, ground and sieved and optionally a surface treatment is applied.
Methods of continuous polymerization are, for example, described in WO-A-01/38402, WO-A-03/004237, WO-A-03/022896 and WO-A-01/16197.
Due to regulations to protect the environment gaseous streams generated in an process for the preparation of superabsorbent polymers have to be purified prior to venting. Thereby waste streams are created that have to be treated and disposed which adds to the overall costs of the process. Furthermore, gaseous streams generated in the process for the preparation of superabsorbent polymers may contain valuable components like monomers or polymer dust and fines, that are lost when dumping said waste streams. Thus, there is a desire in industry to utilize such waste streams without compromising the efficiency of the overall process especially the product quality.
WO-A-03/022896 discloses a continuous process for the preparation of superabsorbent polymers in a reactor system comprising at least 3 zones, wherein the first zone is an initiation zone to which an aqueous monomer mixture comprising an initiation system is fed under conditions to initiate the polymerization, the second zone is a gel-phase zone and the third zone is a granulation zone. According to a preferred embodiment, the reactor is operated under vacuum and any water contained in the vacuum line is condensed and the condensate is recycled to the third zone to enhance the flowability of the polymer gel in the third zone. WO-A-03/022896 is silent of any other components contained in the condensate. Since the process is conducted in an aqueous system water is a neutral component in the process. Thus, WO-A-03/022896 does not suggest that waste streams containing numerous impurities can be utilized without compromising the overall efficiency of the process.
Thus, the object of the present invention is to provide a process for the preparation of superabsorbent polymers, wherein waste streams generated in the process can be utilized without impairing the product quality.